The present invention relates to a novel and improved key-board switch unit or, more particularly, to a structure of a key-board switch unit of which the assembling work of parts can be performed very conveniently and efficiently.
As a key-board switch unit for a complicated electric circuit such as those shown by an equivalent circuit of a sequence of so-called X-Y matrix, there is known a switch unit comprising a printed circuit board provided with a plurality of fixed contact points, fixed terminals and lead-out terminals as well as wiring circuits connecting them, a spacer sheet placed on the printed circuit board and provided with openings at positions corresponding to the fixed contact points and fixed terminals on the printed circuit board and an elastically resilient and electrically insulating covering sheet member placed on the spacer sheet and provided on the surface contacted with the spacer sheet with flexible contact points facing the fixed contact points across the openings in the spacer sheet. The flexible contact points on the covering sheet member are formed, for example, by printing with an electroconductive paint or printing ink and connecting circuits between them are simultaneously formed according to the desired circuit diagram. When the printed circuit board and the covering sheet member is contacted with a fixed terminal on the printed circuit board across an opening in the spacer sheet so that any troublesome means for forming a jumping circuit on the printed circuit board can be obviated.
Key-board switch units of the above described type are advantageously used in a control or operation panel of a variety of electronic instruments taking advantages of their very small thickness and inexpensiveness by virtue of the absence of jumping circuits and throughhole circuits which cost so much in forming.
In mounting the key-board switch unit of the above described type on an electronic instrument, it is a usual practice that several screw bolts are screwed into the frame of the instrument penetrating the key-board switch unit composed of the printed circuit board, spacer sheet and covering sheet member so as that fastening of these three components and mounting of the thus integrated switch unit on the instrument can be simultaneously obtained.
A problem in fastening and mounting of the key-board switch unit in the above described manner is that, because the screw bolts usually penetrate the switch unit at positions apart from the fixed terminals, the contacting condition for making electric connection between the fixed terminal and a part of the connecting circuit on the covering sheet member is sometimes unstable and unreliable due to the distortion caused in the printed circuit board or the covering sheet member depending on the fastening force caused by screwing the screw bolts into the frame.
In order to overcome the above problem and to obtain more reliable contacting condition between the fixed terminals and the connecting circuits on the covering sheet member, there has been proposed a means in which a resilient member such as a coil spring or a rubber pad is applied separately to the switch unit at the position of the fixed terminal. This approach is, however, not practical because the work of assembling and mounting such a key-board switch unit is very troublesome with one or more of the resilient adapter members outwardly applied.